[PATCH 1/2] devicetree/bindings: display: Add bindings for LVDS panels

[Laurent Pinchart]

Hi Rob,

On Friday 14 Oct 2016 07:40:14 Rob Herring wrote:
> On Sun, Oct 9, 2016 at 11:33 AM, Laurent Pinchartwrote:
> > On Saturday 08 Oct 2016 20:29:39 Rob Herring wrote:
> >> On Tue, Oct 04, 2016 at 07:23:29PM +0300, Laurent Pinchart wrote:
> >>> LVDS is a physical layer specification defined in ANSI/TIA/EIA-644-A.
> >>> Multiple incompatible data link layers have been used over time to
> >>> transmit image data to LVDS panels. This binding supports display
> >>> panels compatible with the JEIDA-59-1999, Open-LDI and VESA SWPG
> >>> specifications.
> >>> 
> >>> Signed-off-by: Laurent Pinchart
> >>> <laurent.pinchart+renesas at ideasonboard.com>
> >>> ---
> >>> 
> >>>  .../bindings/display/panel/panel-lvds.txt          | 119 +++++++++++++
> >>>  1 file changed, 119 insertions(+)
> >>>  create mode 100644
> >>>  Documentation/devicetree/bindings/display/panel/panel-lvds.txt>
> >>> 
> >>> diff --git
> >>> a/Documentation/devicetree/bindings/display/panel/panel-lvds.txt
> >>> b/Documentation/devicetree/bindings/display/panel/panel-lvds.txt new
> >>> file
> >>> mode 100644
> >>> index 000000000000..250861f2673e
> >>> --- /dev/null
> >>> +++ b/Documentation/devicetree/bindings/display/panel/panel-lvds.txt
> >>> @@ -0,0 +1,119 @@
> >>> +Generic LVDS Panel
> >>> +==================
> >>> +
> >>> +LVDS is a physical layer specification defined in ANSI/TIA/EIA-644-A.
> >>> Multiple
> >>> +incompatible data link layers have been used over time to transmit
> >>> image data
> >>> +to LVDS panels. This bindings supports display panels compatible with
> >>> the
> >>> +following specifications.
> >>> +
> >>> +[JEIDA] "Digital Interface Standards for Monitor", JEIDA-59-1999,
> >>> February
> >>> +1999 (Version 1.0), Japan Electronic Industry Development Association
> >>> (JEIDA)
> >>> +[LDI] "Open LVDS Display Interface", May 1999 (Version 0.95), National
> >>> +Semiconductor
> >>> +[VESA] "VESA Notebook Panel Standard", October 2007 (Version 1.0),
> >>> Video
> >>> +Electronics Standards Association (VESA)
> >>> +
> >>> +Device compatible with those specifications have been marketed under
> >>> the
> >>> +FPD-Link and FlatLink brands.
> >>> +
> >>> +
> >>> +Required properties:
> >>> +- compatible: shall contain "panel-lvds"
> >> 
> >> Maybe as a fallback, but on its own, no way.
> > 
> > Which brings an interesting question: when designing generic DT bindings,
> > what's the rule regarding
> 
> Call it "simple" so I can easily NAK it. :)
> 
> Define a generic structure, not a single binding trying to serve all.
> 
> >>> +- width-mm: panel display width in millimeters
> >>> +- height-mm: panel display height in millimeters
> >> 
> >> This is already documented for all panels IIRC.
> > 
> > Note that this DT binding has nothing to do with the simple-panel binding.
> > It is instead similar to the panel-dpi and panel-dsi-cm bindings (which
> > currently don't but should specify the panel size in DT). The LVDS panel
> > driver will *not* include any panel-specific information such as size or
> > timings, these are specified in DT.
> 
> The panel bindings aren't really different. The biggest difference was
> location in the tree, but we now generally allow panels to be either a
> child of the LCD controller or connected with OF graph. We probably
> need to work on restructuring the panel bindings a bit. We should have
> an inheritance with a base panel binding of things like size, label,
> graph, backlight, etc, then perhaps an interface specific bindings for
> LVDS, DSI, and parallel, then a panel specific binding. With this the
> panel specific binding is typically just a compatible string and which
> inherited properties apply to it.
> 
> >>> +- data-mapping: the color signals mapping order, "jeida-18",
> >>> "jeida-24"
> >>> +  or "vesa-24"
> >> 
> >> Maybe this should be part of the compatible.
> > 
> > I've thought about it, but given that some panels support selecting
> > between multiple modes (through a mode pin that is usually hardwired), I
> > believe a separate DT property makes sense.
> 
> Okay.
> 
> > Furthermore, LVDS data organization is controlled by the combination of
> > both data-mapping and data-mirror. It makes little sense from my point of
> > view to handle one as part of the compatible string and the other one as
> > a separate property.
> > 
> >>> +Optional properties:
> >>> +- label: a symbolic name for the panel
> >> 
> >> Could be for any panel or display connector.
> > 
> > Yes, but I'm not sure to understand how that's relevant :-)
> 
> Meaning it should be a common property.
> 
> >>> +- avdd-supply: reference to the regulator that powers the panel
> >>> analog supply
> >>> +- dvdd-supply: reference to the regulator that powers the panel
> >>> digital supply
> >> 
> >> Which one has to be powered on first, what voltage, and with what time
> >> in between? This is why "generic" or "simple" bindings don't work.
> > 
> > The above-mentioned specifications also define connectors, pinouts and
> > power supplies, but many LVDS panels compatible with the LVDS physical
> > and data layers use a different connector with small differences in power
> > supplies.
> > 
> > I believe the voltage is irrelevant here, it doesn't need to be controlled
> > by the operating system. Power supplies order and timing is relevant,
> > I'll investigate the level of differences between panels. I'm also fine
> > with dropping those properties for now.
> 
> Whether you have control of the supplies is dependent on the board.
> Dropping them is just puts us in the simple binding trap. The simple
> bindings start out that way and then people keep adding to them.
> 
> >>> +- data-mirror: if set, reverse the bit order on all data lanes (6 to 0
> >>> instead
> >>> +  of 0 to 6)
> 
> On this one, make the name describe the order. "mirror" requires that
> I know what is normal ordering. Perhaps "data-msb-first".

The normal order is defined just below in the same document, and actually 
transmits bits with MSB first (roughly). The bit ordering in LVDS is a bit 
messed up, I've tried to follow the specs as much as possible for these 
bindings. Data mirroring isn't defined in any spec I've found but is 
implemented by some devices that got the spec wrong :-)

-- 
Regards,

Laurent Pinchart